1. FIELD OF THE INVENTION
The present invention relates generally to a cooling device for internal combustion engines. More particularly, it relates to a device for cooling pistons in internal combustion engines by means of sprayed lubricating oil.
2. DESCRIPTION OF THE RELATED ART
It is desirable for pistons in internal combustion engines to be cooled by lubricating oil or the like. Cooling the pistons increases its torque and prevents the engine from knocking, resulting in improved fuel consumption. One such type of cooling systems sprays lubricating oil on the rear surface of the piston. In such a system, oil passages run through the crank shaft and connecting rod to the piston for delivering the lubricating oil.
One such lubricating oil supplying structure is disclosed in Japanese Laid-Open Utility Model Publication No. 58-106612. As shown in FIG. 5, this structure has an oil passage 105 running from a main journal 109 in a crank shaft 115 to a crank pin 103. Ports 106 are provided on a metal bearing 102, and an oil passage 104 extends along a longitudinal axis of a connecting rod 101. The passages 104 and 105 and the ports 106 communicate with one another as shown in the drawing. In FIG. 5, the rotating direction of the crank shaft 115 is clockwise.
According to the foregoing structure, the lubricating oil for cooling the piston is led through a running route described below and is supplied and sprayed at least once each cycle. In successive cycles, the supplying and spraying are repeated in the same manner.
The lubricating oil is delivered to a bearing (not shown) in the main journal 109 of the crank shaft 115 by means of an oil pump. After the lubricating oil lubricates the bearing, the lubricating oil is led into an oil passage 113 provided on the periphery of the main journal 109. The oil passage 113 communicates with the oil passage 105. The oil passage 105 communicates with the outside by way of a bore 108 defined on the periphery of the crank pin 103. Accordingly, the lubrication oil always remains in a clearance between the crank pin 103 and the bearing 102.
The bearing 102 is fixed to the connecting rod 101, and the crank pin 103 moves relative to the bearing. When the crank pin 103 comes to a position shown in FIG. 5, the bore 108 corresponds to and communicates with one of the ports 106. A part of the lubricating oil supplied from the bore 108 remains in the clearance to form an oil film, but most of the oil is led through a notch 107 into passage 104. The lubricating oil in the passage 104 is then delivered to a notch 112 formed at a distal end portion of the connecting rod 101 and stays at a position adjacent to a bore 110.
Then, when the piston 116 reaches the top dead center, and turns downward, the inertia of the lubricating oil is generally directed upwards. The lubricating oil can not stay in the oil passage 104 due to the inertial force and is thus sprayed towards the rear surface of the piston 116. Especially after the piston 116 passes the compression top dead center, the combustion force bears more downward acceleration upon the connecting rod 101. Accordingly, the upward inertia force upon the lubricating oil also increases, resulting in a larger pressure for spraying the lubricating oil which now acts as a coolant. When both the ports 106 and 108 communicate with each other again in the next cycle, the lubricating oil is supplied into the oil passage 104 in the connecting rod 101 in the same manner and is sprayed again immediately after the piston 116 passes the top dead center. The foregoing cycle is repeated as described above, so that the piston 116 is cooled when the engine works.
As shown in FIG. 5, there a clearance is provided between the bearing 102 and the crank pin 103, to receive oil therein. When the engine is at work, the lubricating oil is supplied from the bore 108 and an oil film is formed in the clearance. This facilitates the lubrication of the sliding surfaces between the crank pin 103 and the bearing 102. Accordingly, it is necessary to supply a predetermined amount of lubricating oil to the clearance to maintain the oil film and achieve the desired lubricating effect.
During the combustion cycle, the center of a large end portion of the connecting rod 101 is likely to be slightly displaced from that of the crank pin 103 due to the inertia loads caused by the up/down and rotating motions of the connecting rod 101. Therefore, a volume of the foregoing clearance often inevitably becomes smaller, resulting in that the oil film is pressed and deformed. The deformation of the oil film in the clearance is described below. In the following description, when the oil film is said to be the thinnest it means that the oil film is deformed the most.
FIG. 6 (b) is a polar coordinate showing the displacements of the center of the proximal end portion of the connecting rod 101 relative to the center of the crank pin 103, having the compression top dead center as a starting point. The coordinate is set as shown in FIG. 6 (a), and an external circle represents the maximum volume of the clearance.
In regions Q and Q' in the graph shown in FIG. 6 (b), the crank angle is in the range between 0 and 90 degrees from the top or bottom dead center. Within these regions Q and Q', the center of the proximal end portion of the connecting rod 101 is displaced from the center of the crank pin 103 the most significantly compared with the other periods in the cycle. For example, during Q the crank pin 103 is located at a far upper right part of the proximal end portion of the connecting rod 101. The reason for the displacement is that the connecting rod 101 and the crank pin 103 tend to move apart from each other just before and after a piston reaches the top or bottom dead center. This is because that at such times the piston 116 tends to move in a first direction because of its inertia while the crank pin 103 turns in the reverse direction.
As the connecting rod 101 and the crank pin 103 move apart from each other at these times, the center of the proximal end portion is displaced from the center of the crank pin 103 the most significantly which in turn causes the thinnest oil film. Moreover, the crank pin 103 rotates relative to the bearing 102, so that the oil film is pressed and deformed between the internal surface of the bearing 102 and the external surface of the crank pin 103. In this condition, the actual position of the thinnest oil layer moves along the internal surface of the bearing 102. Accordingly, the oil film is very rapidly pressed and deformed, so that there is no enough time for all the lubricating oil forming the oil film to move into a wider area within the clearance. Rather, a considerable amount falls off from side edges of the bearing 102. Accordingly, sufficient new lubricating oil must be supplied to the clearance to compensate for oil that falls away due to the deformation.
However, in the lubricating oil supplying device disclosed in Japanese Laid-Open Utility Model No. 58-106612, the port 106 and bores 108 are aligned and communicate with each other just after the piston passes the top dead center as shown in FIG. 5. Accordingly, even though the amount of lubricating oil falling off from the side edges of the bearing 102 is relatively large just after the piston reaches the top dead center, at that time, most of the lubricating oil is delivered into the oil passage 104 in the connecting rod 101. Consequently, insufficient lubricating oil may be supplied to the clearance which prevents the oil film from being formed and causes a problem due to a lack of lubrication between the bearing 102 and the crank pin 103.